docs: server-render a useful 404 page on both sites

[vihar]

Summary

Item 3 from the is-agentic scan (after #508, #509; independent of both).

Problem. Unknown paths on both sites return a real 404 status, but the body is an empty app shell — VitePress deliberately writes <div id="app"></div> into the 404.html it generates (even with a custom 404.md; hasCustom404 only affects preload links). Agents and no-JS clients get nothing to recover from, and humans only see content after JS loads:

$ curl -s https://docs.plane.so/does-not-exist | grep -o '<div id="app">[^<]*'
<div id="app">

Fix. One shared component, rendered in two places so humans and agents always see the same thing:

• packages/theme/src/components/NotFound.vue — "Page not found" with where-to-look-next links: home, search (⌘K), /sitemap.xml, /llms.txt, /llms-full.txt, the other Plane docs site (so someone looking for the API on docs.plane.so gets pointed at developers.plane.so and vice versa), a help link, and a one-line hint for agents (append .md or send Accept: text/markdown). Registered as <PlaneNotFound> and used as the Layout's default not-found slot for client-side navigation to a missing page. Site-specific wording comes from a new notFound option on createPlaneTheme.
• apps/*/docs/not-found.md renders the component as a normal page so VitePress fully server-renders it; each app's existing buildEnd then moves not-found.html over 404.html and removes the stray not-found.* copies. The page is noindex, search: false, copyPage: false, excluded from llms.txt and filtered out of the sitemap.

Both sites are configured identically apart from the wording (site name, sibling site, help link).

Verification

• pnpm check:types, check:format, and pnpm build pass.
• In both dist/s: 404.html contains the full rendered content and all links; <title>Page not found | Plane</title>; robots: noindex, nofollow; not-found/404 absent from sitemap.xml, llms.txt, llms-full.txt; no leftover not-found.html/.md.
• After deploy: curl -s https://docs.plane.so/does-not-exist | grep -c "Where to look next" → 1 (and status stays 404).

Checklist

• 1. Markdown negotiation — fix: serve markdown via routing middleware for Accept: text/markdown #508
• 2. Canonical + Organization JSON-LD — docs: add canonical links and Organization JSON-LD to both sites #509
• 3. Useful 404 page — this PR
• 4. llms.txt improvements (cross-site links, ## Optional, dedupe) — next
• 5. openapi.json on developers.plane.so — blocked on a spec source
• 6. Re-run npx is-agentic on both hosts

🤖 Generated with Claude Code

https://claude.ai/code/session_01CsPSwTnpsEb5c5Ud2CLrL8

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Project	Deployment	Actions	Updated (UTC)
developer-docs	Ready	Preview	Aug 21, 2026 8:27pm
docs	Ready	Preview	Aug 21, 2026 8:27pm

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 51b60389-5aaf-4628-8b4c-87be0c900b0c

📥 Commits

Reviewing files that changed from the base of the PR and between aad24bb and 79dd59d.

📒 Files selected for processing (13)

• apps/developer-docs/AGENTS.md
• apps/developer-docs/docs/.vitepress/config.mts
• apps/developer-docs/docs/.vitepress/theme/index.ts
• apps/developer-docs/docs/not-found.md
• apps/docs/AGENTS.md
• apps/docs/docs/.vitepress/config.ts
• apps/docs/docs/.vitepress/theme/index.ts
• apps/docs/docs/not-found.md
• packages/theme/README.md
• packages/theme/src/components/NotFound.vue
• packages/theme/src/index.ts
• packages/theme/src/layout/Layout.vue
• packages/theme/src/options.ts

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